Thread-forming screw

Abstract

A thread-forming screw includes a tapered end portion formed by inserting a cylindrical blank between paired dies each having a straight portion and a tapered portion. The threads of each die are provided with undulations along their lengths corresponding to the circumference of the screw blank so as to form a trilobular thread on the screw which spirals around its circumference from one end of the screw to the other in a direction opposite to that of the threads. The cylindrical blank provides increased material at the tapered thread-forming location for forming fully crested threads, while increasing the length of the tapered portion of the die and screw provides greater stress relief for reducing the force required for thread formation and allows the screw to be driven into a workpiece more easily while maintaining a high strip force. In a preferred embodiment, the tapered end of the screw has a constant minor diameter and two pitches in which the major diameter diminishes toward the end followed by two end pitches in which the major diameter remains constant.

I claim: 
     
       1. A thread-forming screw comprising: an elongated shank having a continuous exterior thread and a plurality of exterior lobes distributed circumferentially about said shank in an equally spaced manner and extending spirally therealong in an angular direction opposite from said thread, said shank having in any plane normal to its axis an inner solid core portion of circular configuration and said lobes projecting outwardly beyond said core portion, said shank including a cylindrical threaded portion and an increased stress relief, tapered threaded end portion, wherein said cylindrical and tapered threaded portions are formed from an unthreaded, untapered cylindrical blank for increased stress relief in forming fully crested threads with the application of reduced torque, wherein said tapered end potion has a constant minor diameter and includes four pitches, with two pitches having a diminishing major diameter toward the end and two end pitches having a constant major diameter for maintaining the screw in alignment with an aperture in which the screw is inserted to facilitate driving the screw into said aperture.